U.S. Military Half-tracks
The primary means of transporting infantry and towed weapons across a battlefield in the U.S. Army is a design series of armored half-tracks. The different models of vehicle share many similar parts and characteristics in order to make repair, maintenance and manufacture as simple as possible. Although new types of sealed and self-contained Armored Personnel Carriers, the most recent iteration of which is the M113, are replacing these vehicles in some roles among Armored and Mechanized units, for the Infantry Divisions - the U.S. Army's backbone - and logistical purposes, these half-tracks are standard. Taken together, they form the most common class of vehicle in American Military service. They are purpose-built military hardware, but used for everything from battlefield infantry support, to towing artillery pieces, to transporting supplies over friendly roads and moving aircraft at airfields - fufilling roles allocated to civilian-manufactured automobiles in most other armies. As such, they have also become a ubiquitous sight within the "Homeland States" of the Continental U.S., and even in her overseas territories as well. Carrier, Personnel, Half-track M77A5 The Carrier, Personnel, Half-track M77A5 is the principle means of motorization for U.S. Infantry Regiments. The M77A5 is an aluminimum-armored steel, all-wheel-drive half-track powered by a Detroit 6V53T 6-cylinder Diesel engine rated for 215 hp. Original versions of the vehicle were open-topped, but as of the A3 model an armored roof has been fitted over the transport compartment here with the large hinged rear section open. The A4 version, identical to the A5 apart from the "Vampire" night vision kit, has a redesigned external body (although the frame, chassis, drivetrain, etc. are identical). The original steel hull of the basic M77 through M77A3 was replaced with new aircraft-quality Aluminum shell in the -A4 model, specifically made of Alloy 5083: a form of Aluminum with 4-5% magnesium, 1% manganese and traces of other minerals such as Chromium, as well as a small quantity of Titanium (~0.15%). This metal - being somewhat lighter than Rolled Homogeneous Armor (RHA) steel but of comparable strength - improves the protection of the vehicle by permitting heavier armor for the same weight. This also makes the M77A4 and -A5 sturdier, because the thicker skin is better able to withstand tension and stress. Rolled Homogeneous Armor steel has a density of 7.84 g/cm^3, while the density of 5083 Aluminum is 2.768 g/cm^3 - making it about 3 (2.81) times lighter. The RHA steel armor of the original M77 through the M77A3 varied in thickness between 9mm / 0.35" and 21mm / 0.83", depending on the plate. As of the -A4 model, the scheme of the new Aluminum armor is as follows (note that angle measurements are from Vertical, i.e. vertical is 0° & horizontal is 90°. The higher the angle number, the closer it is to horizontal.): Upper Front - 45mm at 33° *note: upper front = sloped plate directly in front of driver's compartment where vision blocks are located Lower Front - 45mm at 21° *note: lower hull front = plate directly in front of the engine, i.e. the actual front of the vehicle Upper Sides - 30mm at 35° Middle Sides - 30mm at 35° *note: middle hull side = narrow strip of armor directly above the tracks that slopes inward rather than outward Lower Sides = 20mm at 0° vertical *note: lower hull side = area behind tracks & behind wheels Forward Sides = 35mm at 35° *note: forward sides = sides of the engine compartment, above the wheels Rear = 30mm at 30° Top = 30mm at 90° horizontal *note: hull top = closed roofed section above the driver's compartment and rim around roof hatch Roof Panels = 35mm at 90° horitzontal Forward Top = 25mm at 76° *note: forward top = steeply-inclined, nearly flat plate over the engine compartment, i.e. the "hood". The armor here is thinner because its steep angle increases the relative effectiveness immensely. Floor / bottom = 25mm at 90° horizontal The M77A4 also has "doubled up", or "dually", front wheels (i.e. two side-by-side wheels on either end of the axle) to decrease ground pressure and reduce the effects from having a tire being blown out. Beginning in 1960, the M77A4 was fitted with the cutting-edge M27A2B1 "Vampire D" system and an extra set of infrared headlights to give it night-fighting capability, creating the current M77A5 model. Each M77A5 is armed with two Browning 12.7mm .50-cal AN/M3 machine guns with a rate of fire of 1,200 rpm, mounted at opposite ends of the transport compartment. One is affixed to the pintle mount of the hatch at the front left of the vehicle, just behind the commander's seat. The other is on a swinging gimble above the rear disembarkation doors and can only be operated when the roof is opened - the weapon is either swung forward or pulled back and down into the vehicle when these large panels are closed. The half-track has a crew of 3: Driver, who sits at the front on the left, a Commander who sits next to the driver and serves as a radio operator, and a crew chief who serves to operate the forward machine gun. The rear machine gun has no dedicated operator and is to be used by the soldiers riding in the vehicle, although in a pinch it can be manned by the M77's commander. The M77A5 has a weight of 13.5 tonnes Carrier, General-purpose, Half-track M78A5 The M78 is a multipurpose utility transport variant of the M77. It serves a number of roles for which the larger, personnel carrier M77 is not suited: ammunition transport, artillery forward observation, headquarters, radio, reconnaissance and such. In almost all respects, it is identical to the M77. The only real difference is the design of the body: the drive train, chassis, suspension are the same, just slightly shorter. The transport bed of the M78 is shorter, and the walls lack the prominent angled "clamshell" shape in order to increase the internal volume (although the actual width of the vehicle is essentially the same). M78s are used primarily for moving around supplies, equipment and ammunition close to (or on) the front lines, because they are better protected than other cargo-oriented half-tracks and more maneuverable (although their carrying capacity is considerably smaller). M78s are also the principle gun tractors for American towed weapon systems like the M117 gun-howitzer. For this reason, M78s are still equipped with bench seats that fold up flush with the sides of the cargo bed, allowing them to carry the crews of the weapons they tow. However, these half-tracks lack the large hinged roof panels of the M77: they are open-topped. The M78's variant scheme is identical to the M77, and the modern -A5 version has received all of the same upgrades, including the Vampire night-vision system. Each M78 is armed with a single .50-calibre machine gun at the front of the transport compartment. However, when the half-track is not carrying passengers, this weapon must be manned by the Commander (who normally rides next to the driver) as the vehicle only has a two-man crew. Carrier, Ammunition, Half-rack M80A2 Close-topped modification of the M78 that serves as a dedicated ammunition carrier. The sealed transport bed of the M79 is better suited for this sort of role, since it protects the vehicle's explosive cargo against shell fragments and stray bullets. M79s are normally paired with a specialized close-topped trailer for additional carrying capacity. The M79 is a more recent design than the M77/78, and as such its variant designation is out of sequence with them. It was introduced around the same time as the -A3 versions of the M77/78 to replace the M78s in their front-line ammunition carrier capacity, since the open-topped bed of those vehicles was rapidly proving a liability. The M79A2 is identical in terms of upgrades / modifications to the -A5 versions of its cousins. Unlike them, however, it is unarmed. Carrier, Cargo, Half-track M80A1 A larger half-track designed to replace the venerable M35 family of 2.5 ton trucks. Like those vehicles it has replaced, the M80 is affectionally referred to by U.S. troops as the "Deuce-and-a-half" - although, in the case of the M80, this refers to 2.5 Metric tonnes rather than 2.5 short tons (the U.S. having converted officially to the Metric system in 1955). Thus, the half-track's carrying capacity is closer to 3 short tons. The M80, unlike the M78 and -79, is not directly derived from the M77, although it still shares a high commonality of parts. It is considerably larger, but uses the same engine (which it also shares with the M113 APC) for the sake of logistical simplicity. This makes it rather less maneuverable than the M77 family, although that's not really a problem because it is not normally employed in front-line operations. It has a top speed of 55km/h (compared to 70km/h for the M77 and its variants). Unlike the M77 family, the M80 is not equipped with Vampire night-vision systems. It does, however, have a single .50-cal machine gun mounted in the front of the transport compartment for use by the vehicle's crew chief, as on the M77. The transport bed of the M80 is open-topped and has single-piece, slightly-sloped sides. Unlike a conventional cargo truck, it is armored in aluminum like the M77, although the plating is somewhat thinner. Tractor, General-purpose, Half-track M8A2 "Rabbit" The M8A2 "Rabbit" is a small multipurpose light tractor, a unique hybrid of half-track & motorcycle. It is a heavily modified version of the U.S. Military's standard Indian-Chrysler 750 motorcycle. They are used for a variety of purposes, such as laying cables, transporting wounded soldiers off the battle line or ferrying important personnel from location to location. It serves in many overlapping capacities with the Indian-Chrysler 750 heavy-duty motorcycle from which it is derived. However, the Rabbit is preferred in inclimate weather conditions or on uneven such as the battlefields of Alaska. In this rough, snow-covered ground, the 750 motorcycle can only really be used over prepared paths, and such is limited to rolls where speed is of necessity (such as courier / message-running work). M8 Rabbits are particularly common among various specialist units, especially the Airborne and Pioneers. They serve the USAAF, Air National Guard and U.S. Marine Coprs Aviation as runway tugs for the venerable P-56 "Screaming Eagle" fighters and as ground-crew transports. They are also used by Infantry Regiments as the standard gun tractor for their light field pieces: the 75mm Regimental Gun M121 & 105mm Infantry Gun M130. Another common function of the Rabbit is to rapidly bring up smallarms ammunition to the front line, which is why the vehicle comes from the factory with a removable steel cowling for its back compartment (to protect cargo from shell fragments or stray bullets) and a small, specially-designed, light-weight two-wheel trailer to increase its carrying capacity (which also has its own detatchable armor plate lid). This trailer is specifically designed to conform to the dimensions of the wooden shipping crates that hold the U.S. Army's standardized aluminum ammunition boxes. One such crate can hold four M19A1 ammunition boxes, each with twenty-four 8-round en bloc clips of 7.62x51mmA cartridges as well as a spare sling and half-dozen standard-issue M1A 25-round box magazines for the M14A3 Service Rifle - thus, a total package of 768 cartridges in 96 clips, 24 M1A magazines for the M14 and 4 Springfield Armory-manufactured slings.